An electromagnetic bandgap can be implemented by periodically arranging desired unit cells on an electric conductor by a preset interval or without the preset interval therebetween, and on the surfaces of arrangements of the unit cells, tangent component of a magnetic field becomes ‘0’ (zero) at a specific band so that an electric current cannot flow on the surfaces of the electromagnetic bandgap. This feature is a concept opposite to that of an electric conductor and is related to a magnetic conductor, and the surfaces of the electromagnetic bandgap, i.e., the surfaces of the arrangements of the unit cells becomes a high impedance surface in view of an electric circuit. Since a feature of a theoretical magnetic conductor, which cannot exist in real situation, is implemented on the surfaces of the electromagnetic bandgap, the theoretical magnetic conductor is known as an artificial magnetic conductor. This structure, in the field of optics, originally coming from photonic bandgap technology invented to prevent an optical wave from advancing at a specific bandwidth in a guided structure, is recently known as an electromagnetic bandgap for a microwave frequency band as a frequency band to which the structure may be applied is becoming more broad, and is chiefly applied to various fields such as an antenna, a filter, a waveguide, and the like.
Since the electromagnetic bandgap is mostly applied to the antenna field, the electromagnetic bandgap can be understood well by an example of an antenna. Generally, in order to radiate electromagnetic waves effectively, an antenna parallel to a ground of an electric conductor requires a distance longer than λ/4 (λ is a wavelength at a resonance frequency) from the ground. When the distance between the antenna and the ground of the electric conductor is shorter than λ/4, since a surface current is induced on a surface of the ground of the electric conductor in the direction opposite to a current flowing in the antenna, the currents cancel each other so that the antenna cannot radiate electromagnetic waves. However, when the electromagnetic bandgap is applied instead of the ground of the electric conductor, since the surface current can be prevented from flowing on surfaces of the electromagnetic bandgap at a specific bandwidth, the antenna can be operated at a position much nearer than that of the antenna on the electric conductor. Thus, the distance from the ground to the antenna can be reduced so that the antenna can be made small.
Since the electromagnetic bandgap interrupts the surface current at a specific bandwidth, undesired radiation of electromagnetic waves generated from an edge of a finite ground due to the surface current can be reduced. Since the electromagnetic waves radiated from the antenna to the ground side are reflected at the same phase as that of electromagnetic waves directly radiated in the opposite direction by the electromagnetic bandgap, back radiation can be reduced and radiation gain in a main beam direction can be improved.
Since the above-described technical features of the electromagnetic bandgap are mainly applied to planar antennas, the electromagnetic bandgap is recently being widely applied as a solution for a small antenna, and for improving isolation characteristics between antennas and radiation characteristics of the electromagnetic waves.
However, the electromagnetic bandgap is not being applied to a base station antenna, a repeater antenna, a satellite-tracking antenna, a vehicle antenna, and the like, yet.